This invention relates in general to reaction engines and, more particularly, to nozzles for such engines.
The typical ramjet engine consists essentially of an inlet, a combustion chamber, a fuel metering apparatus for supplying fuel to the combustion chamber where it burns to produce elevated pressures, and a converging-diverging nozzle for converting the elevated pressure within the combustion chamber into useful thrust. Since ramjet engines are capable of operating at supersonic speeds they are particularly useful for propelling high speed aircraft.
Like any other aircraft engine, a ramjet engine must accommodate various conditions of flight, such as the high thrust required for acceleration and climb, the lower thrust required for high altitude cruising, and again the high thrust required for approach to the final destination at lower elevations. Since a ramjet engine operates most efficiently when the pressure within its combustion chamber is maximum-that maximum being determined by the configuration of the inlet to the engine-a single nozzle size will not maintain the maximum pressure for both the high thrust conditions of climb, where the specific fuel consumption is high, and the relatively low thrust required for cruising at high elevation, where the specific fuel consumption is low.
To maintain the high pressure needed for efficient engine operation under both high thrust and low thrust conditions, swing disc nozzles have been employed (See FIG. 1). Basically, these nozzles have the usual converging-diverging configuration which is capable of accelerating pressurized combustion gases generated ahead of it to supersonic velocities. A disc which is somewhat smaller than the throat of the nozzle is mounted in the nozzle on trunnions so that it can turn from a position parallel to the axis of the nozzle (FIGS. 1a & 1b) to a position perpendicular to the axis (FIGS. 1c & 1d). In the former the nozzle throat is practically unobstructed, and the engine will efficiently delive a large amount of thrust, assuming that sufficient fuel is supplied to the combustion chamber. On the other hand, in the latter or perpendicular position, the nozzle throat is actually an annulus surrounding the closed disc. This maintains the combustion chamber pressure relatively high when fuel is supplied to the chamber at a lesser rate and consequently the engine produces less thrust, which is desired for cruise conditions in the rarefied atmosphere of high altitudes. Also, because of the smaller throat area, the combustion gases experience a greater expansion between the throat and the nozzle exit. The velocity of the gases in the nozzle is therefore greater, and this improves the efficiency of the nozzle during cruise conditions.
While the conventional swing disc nozzle improves the efficiency of the engine operation at cruise conditions, operating under these conditions is still less than ideal. This derives from the fact that the closed disc around which the gases pass as they discharge from the nozzle produces aerodynamic drag. The drag or turbulence in turn reduces the efficiency of the engine, although the efficiency is still higher than it would be if the disc were not present.
Heretofore some engines have been provided with multiple nozzles, instead of a single large nozzle, the primary purpose of this arrangement being to reduce the overall length of the nozzle region and thereby make the engine more compact. However, nozzles of these types have not been modulated to accommodate the high thrust demands of climb and the low thrust demands of cruise.